Anaerobic fermentations of hydrogen and carbon monoxide involve the contact of the substrate gas in a liquid aqueous menstruum with microorganisms capable of generating oxygenated organic compounds such as ethanol, acetic acid, propanol and n-butanol. The production of these oxygenated organic compounds requires significant amounts of hydrogen and carbon monoxide. For instance, the theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O→C2H5OH+4CO2 6H2+2CO2→C2H5OH+3H2O.
As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion.
Typically the substrate gas for carbon monoxide and hydrogen conversions is or is derived from a synthesis gas (syngas) from the gasification of carbonaceous materials, reforming of natural gas and/or biogas from anaerobic fermentors or from off streams of various industrial methods. The gas substrate contains carbon monoxide, hydrogen, and carbon dioxide and usually contains other components such as water vapor, nitrogen, methane, ammonia, hydrogen sulfide and the like. (For purposes herein, all gas compositions are reported on a dry basis unless otherwise stated or clear from the context.)
Syngas fermentation processes suffer from the poor solubility of the gas substrate, i.e., carbon dioxide and hydrogen, in the liquid phase of the aqueous menstruum. Munasinghe, et al., in Biomass-derived Syngas Fermentation in Biofuels: Opportunities and Challenges, Biosource Technology, 101 (2010) 5013-5022, summarize volumetric mass transfer coefficients to fermentation media reported in the literature for syngas and carbon monoxide in various reactor configurations and hydrodynamic conditions. A number of conditions can enhance the mass transfer of syngas to the liquid phase. For instance, increasing the interfacial area between the gas feed and the liquid phase can improve mass transfer rates.
Numerous processes have been disclosed for the conversion of carbon monoxide and hydrogen to oxygenated compounds. One such process suspends the microorganisms for the conversion in an aqueous menstruum contained in a stirred tank reactor such as by using a motor driven impeller. Stirred tank fermentation reactors provide many advantages. For stirred tank reactors, increasing the agitation of the impeller is said to improve mass transfer as smaller bubble sizes are obtained. Also, the stirring action not only distributes the gas phase in the aqueous menstruum but also the duration of the contact between the phases can be controlled. Another very significant benefit is that the composition within the stirred tank can be relatively uniform. For instance, Munasignhe, et al., in a later published paper, Syngas Fermentation to Biofuel: Evaluation of Carbon Monoxide Mass Transfer Coefficient (kLa) in Different Reactor Configurations, Biotechol. Prog., 2010, Vol. 26, No. 6, pp 1616-1621, combine a sparger (0.5 millimeter diameter pores) with mechanical mixing at various rotational rates to provide enhanced mass transfer. This uniformity enables good control of the fermentation process during steady-state operation. This is of particular advantage in the anaerobic conversion of carbon monoxide and hydrogen to oxygenated compounds where two conversion pathways exist. Hence the carbon dioxide generated from the conversion of carbon monoxide is proximate in location to the hydrogen consumption pathway that consumes carbon dioxide. The uniformity further facilitates the addition of fresh gas substrate. The problems with stirred tank reactors are capital costs, the significant amount of energy consumed in the needed mixing and agitation, and the need for plural stages to achieve high conversion of substrate.
Bredwell, et al., in Reactor Design Issues for Synthesis-Gas Fermentations, Biotechnol. Prog., 15 (1999) 834-844, assessed various types of reactors including bubble columns and stirred reactors. The authors disclose using microbubble sparging with mechanical agitation. At page 839 they state:                “When microbubble sparging is used, only enough power must be applied to the reactor to provide adequate liquid mixing. Thus axial flow impellers desinged to have low shear and a high pumping capacity would be suitable when microbubbles are used in stirred tanks.”They conclude by stating:        “An improved ability to predict and control coalescence rates is needed to rationally design commercial-scale bioreactors that employ microbubble sparging.”        
Another type of fermentation apparatus is a bubble column fermentation reactor wherein the substrate gas is introduced at the bottom of the vessel and bubbles through the aqueous menstruum (“bubble reactor”). See Munasinghe, et al., in Biomass-derived Syngas Fermentation in Biofuels: Opportunities and Challenges, Biosource Technology, 101 (2010) 5013-5022. In order to achieve sought mass transfer from the gas to liquid phases, workers have provided the gas feed to bubble columns in the form of microbubbles. The authors report that in one study, the mass transfer obtained for a bubble column reactor was higher than that for a stirred tank reactor mainly due to the higher interfacial surface area obtained with the bubble column reactor. Advantageously, commercial-scale bubble column fermentation reactors are relatively simple in design and construction and require relatively little energy to operate.
In co-pending U.S. patent application Ser. No. 13/243,426, filed on even date herewith, processes are disclosed for enhancing the performance of large-scale, anaerobic fermentors. In these processes, a reactor having an aqueous menstruum depth of at least about 10 meters is used, and gas feed is supplied to the aqueous menstruum in the form of a stable gas-in-liquid dispersion. The aqueous menstruum is mechanically stirred at a rate sufficient to provide relatively uniform liquid phase composition within the aqueous menstruum without unduly adversely affecting the gas-in-liquid dispersion. For purposes herein the type of stirred tank reactor used in the processes of this invention is called a mechanically-assisted liquid distribution tank reactor, or MLD tank reactor. With the relatively uniform composition throughout the MLD tank reactor provided by the mechanical stirring, regardless of where the gas feed is introduced, bubbles will be moved throughout the volume of the aqueous menstruum. At least a portion of the off-gas from the aqueous menstruum is recycled to obtain a conversion efficiency of total hydrogen and carbon monoxide in the gas substrate to oxygenated organic compound of at least about 80 mole percent in a single reactor stage. Accordingly, capital cost savings and energy savings can be achieved using a MLD tank reactor as compared to a conventional stirred tank reactor.
For purposes herein, both deep, bubble column fermentation reactors and the large-scale liquid mixing reactors supplied with stable gas feed-in-liquid dispersions and using low stirring rates, are referred to as deep, tank reactors.
Deep, tank reactors using microbubbles can provide economically attractive facilities for anaerobic conversion of syngas to oxygenated organic compound, but difficulties are present. In their earlier review article, Munasignhe, et al., report that the gas-liquid mass transfer is the major resistance for gaseous substrate diffusion. The authors state at page 5017:                “High pressure operation improves the solubility of the gas in the aqueous phase. However, at higher concentrations of gaseous substrates, especially CO, anaerobic microorganisms are inhibited.”        
Other workers have understood that the presence of excess carbon monoxide can adversely affect the microorganisms and their performance. See paragraphs 0075 through 0077 and 0085 though 0086 of United States published patent application No. 20030211585 (Gaddy, et al.) disclosing a continuously stirred tank bioreactor for the production of ethanol from microbial fermentation. At paragraph 0077, Gaddy, et al., state:                “The presence of excess CO unfortunately also results in poor H2 conversion, which may not be economically favorable. The consequence of extended operation under substrate inhibition is poor H2 uptake. This eventually causes cell lysis and necessary restarting of the reactor. Where this method has an unintended result of CO substrate inhibition (the presence of too much CO for the available cells) during the initial growth of the culture or thereafter, the gas feed rate and/or agitation rate is reduced until the substrate inhibition is relieved.”        
At paragraph 0085, Gaddy, et al., discuss supplying excess carbon monoxide and hydrogen. They state:                “A slight excess of CO and H2 is achieved by attaining steady operation and then gradually increasing the gas feed rate and/or agitation rate (10% or less increments) until the CO and H2 conversions just start to decline.”        
For deep, tank reactors, the height of the aqueous menstruum is a primary determinant of the contact time for the bioconversion to occur. This height also is a determinant of the static head at the bottom portion of the reactor. Higher pressures result in smaller bubble sizes and higher partial pressures both of which enhance mass transfer efficiency and gas substrate conversion efficiency in the fermentation reactor. Thus, on a commercial scale, deep, tank reactors have a depth of at least about 10, preferably at least about 15, meters and use microbubbles of gas feed in order to achieve conversion efficiencies of at least about 60 percent of the total moles of hydrogen and carbon monoxide supplied to the reactor. However, these operating parameters increase the risk of carbon monoxide inhibition.
For a syngas to oxygenated organic compound fermentation process to be commercially viable, capital and operating costs must be sufficiently low that it is at least competitive with alternative biomass to oxygenated organic compound processes. For instance, ethanol is commercially produced from corn in facilities having name plate capacities of over 100 million gallons per year. Accordingly, the syngas to oxygenated organic compound fermentation process must be able to take advantage of similar economies of scale. Thus, a commercial scale facility may require at least 20 million liters of fermentation reactor capacity. Deep, tank reactors, i.e., reactors having heights of at least 10 meters such as bubble column reactors and mechanically-mixed tank reactors, are attractive for commercial operations due to their large volumes and low capital costs. As reported by Munasignhe, et al., syngas fermentation reactor types such a bubble column reactors and air lift (jet loop) reactors are less costly to manufacture and operate yet can provide good mass transfer rates of syngas to the liquid phase.
In addition to economies of scale, the processes need to obtain high conversion efficiencies of the syngas to oxygenated organic compounds. Syngas and other carbon monoxide and hydrogen-containing gas feeds are typically more expensive than equivalent heat content amounts of fossil fuels. Hence, a desire exists to use these gases effectively to make higher value products. The financial viability of any conversion process, especially to commodity chemicals such as ethanol and acetic acid, will be dependent upon the efficiency of conversion efficiency of the carbon monoxide and hydrogen, the conversion selectivity to the sought products and the energy costs to effect the conversion.
The gas feed can be introduced at the bottom of a deep, tank reactor where the most favorable conditions for mass transfer of carbon monoxide from the gas to the liquid phase exist. Hence, to avoid carbon monoxide inhibition, operating parameters such as carbon monoxide mole fraction in the gas feed, gas feed supply rate and microbubble size must be controlled to assure that the carbon monoxide mass transfer rate does not become so great as to cause carbon monoxide inhibition. However, the conditions required to avoid carbon monoxide inhibition in a deep, tank reactor negatively affect the overall amount of carbon monoxide that can be transferred to the liquid phase, and thus the conversion efficiency to oxygenated organic compound.
This negative effect is particularly exacerbated for deep, bubble column reactors since the static pressure is reduced as the microbubbles pass upwardly, the partial pressure of carbon monoxide in the bubbles decreases and the surface area to volume of the microbubbles may decrease. Furthermore, the compositions of the gas and liquid phases change over the height of the aqueous menstruum, further negatively affecting mass transfer of hydrogen and carbon monoxide to the liquid phase. Carbon dioxide co-product is generated by the carbon monoxide pathway and the solubility of carbon dioxide in the aqueous menstruum is highly sensitive to pressure. Thus, at higher elevations of the aqueous menstruum, carbon dioxide can pass to the bubbles and reduce the mole fractions of hydrogen and carbon monoxide in the gas phase. The reduced mole fractions also reduce the driving force for mass transfer of carbon monoxide and hydrogen to the liquid phase.
The net result is that conversion efficiencies, especially for hydrogen, in a deep, tank reactor are often low. Increasing the depth to provide a longer contact time provides ever diminishing benefits, increases the risk of carbon monoxide inhibition and thus is not a solution by itself to obtain sought high bioconversion efficiencies.
Accordingly, processes are sought that can take advantage of the benefits of deep, tank reactors using small bubbles without undue risk of carbon monoxide inhibition.